The present invention relates to a drawing installation for a press with a mechanically actuated drawing ram and with a sheet metal holder whose holding force acting on the sheet metal member opposite the drawing direction is produced by way of a pressure-medium-actuated piston rod of a pressure cylinder fixed in the press, and in which the piston rod is operatively connected with a pressure piston having an operating surface adapted to be actuated in the drawing direction, and in which a control system is provided operating in dependence on the ram travel.
During the working stroke of the drawing ram of a press, at first the sheet metal member inserted into the work tool is pressed by the drawing ring against the sheet metal holder and thereafter is displaced together with the sheet metal holder opposite the holding force produced by the drawing installation. The sheet metal member held between the sheet metal holder and the drawing ring is deformed on the press table by the drawing die of the work tool half. In addition to the holding force to be produced by the drawing installation, the drawing ram must also overcome the mass inertia forces which become effective at the beginning of the deformation as a result of the acceleration of the movable sheet metal holder and drawing installation masses.
For avoiding the load on the drawing ram which occurs thereby in an impact-like manner, it is known in the drawing installation according to the EP O 074 421 A1 to move the pressure plate carrying several pressure rods for the sheet metal holder by way of a piston rod of a pressure cylinder fixed in the press, which piston rod is adapted to be actuated by a pressure medium. The piston rod is operatively connected with a pressure piston which has an operating surface adapted to be acted upon in the drawing direction. The operating surface is adapted to be acted upon with pressure by way of a control system operating as a function of ram travel so that prior to the contact of the drawing ram on the sheet metal holder, the latter is adapted to be pre-accelerated in the drawing direction. The pressure space coordinated to the operating surface is supplied directly from the pan by way of a motor-pump-aggregate. This aggregate must produce the pressure required for the pre-acceleration against the action of the drawing apparatus. The hydraulic liquid for the main pressures of the drawing, possibly of the blocking of the drawing installation and of the pre-acceleration and the hydraulic liquid for the control are taken from one and the same pan. The movement of the piston rod of the drawing installation is time-controlled during the pre-acceleration phase. The pre-acceleration phase must terminate in time when reaching the maximum deformation movement (lower dead-center point) of the drawing ram.
In contrast thereto, it is the object of the present invention to control the pre-acceleration of the piston rod in the drawing direction by way of a deliberately predetermined (metered) quantity in hydraulic liquid and to control the refill in hydraulic liquid during the further movement of the piston rod in the drawing direction from a reservoir.
The underlying problems are solved according to the present invention in that the pressure space coordinated to the operating surface of the pressure piston is in fluid communication, on the one hand, with the pressure space of a pre-acceleration cylinder consisting of pressure quantity space, pressure space, and a pressure piston separating these spaces from one another, whereby the pressure space is adapted to be actuated with pressure by the control system operating in dependence on ram travel by way of a throttling directional control valve and, on the other, is adapted to be brought into fluid communication with a reservoir for hydraulic liquid at the termination of the pre-acceleration of the pressure piston by way of a follow-up control operating as a function of ram travel.
The relative movement between the drawing ram and the sheet metal holder during the working stroke of the drawing ram can be controlled toward a value=0 and the contact impact can be completely precluded. As a result thereof, pressure vibrations between the sheet metal member and the sheet metal holder are avoided. The sheet metal member is retained uniformly already at the beginning of the drawing phase.